Normally, light waves vibrate in a large number of planes about the axis of a light beam. If the waves vibrate in one plane only, the light is said to be plane polarized. Several useful optical ends and effects can be accomplished by plane polarized light. For example, in the manufacture of electrooptical devices, such as liquid crystal display screens, crossed polarizers are used in conjunction with an addressable liquid crystal interlayer to provide the basis for image formation. In the field of photography, polarizing filters have been used to reduce glare and the brightness of specular reflection. Polarizing filters (circular or otherwise) have also been used for the reduction of glare on CRT display monitor screens.
While several materials possess to a degree inherent polarizing properties, synthetic polarizing materials based on thin polymeric films are desirable for their comparative ease of manufacture and handling, their ability to be tailored for particular uses, and the comparative ease with which they may be incorporated into desired end products.
The production of linear light polarizing films has been well described in the art. Linear light polarizing films, in general, owe their properties of selectively passing radiation vibrating along a given electromagnetic radiation vector (and absorbing electromagnetic radiation vibrating along a second given electromagnetic radiation vector) to the anisotropic character of the transmitting film medium.
Dichroic polarizers are linear polarizers of an absorptive variety that owe their light-polarizing capabilities to the vectorial anisotropy of their absorption of incident light waves. The term "dichroism" is used herein as meaning the property of differential absorption of the components of an incident beam of light, depending upon the vibration directions of said component. Thus, light entering a dichroic film encounters two different absorption coefficients--one low and one high. The emerging light vibrates predominantly in the direction of low absorption.
The most widely used type of synthetic dichroic sheet polarizer is the polyvinyl alcohol-iodine complex polarizer (cf., "H-Sheet"-type polarizer) and variants thereof, the first such polarizer having been invented by Edwin H. Land of Polaroid Corporation (U.S. Pat. No. 2,454,515, issued Nov. 23, 1948on an application filed Oct. 29, 1938). In general, an "H-sheet"-type polarizer comprises a light-absorptive linear polyiodide contained within a polyvinyl alcohol matrix. "H-sheet"-type polarizers are generally made, for example, by impregnating a film of polyvinyl alcohol (or its derivative) with an aqueous solution of a light-absorptive polyiodide (or like dichroic dye) and then thermally stretching the film several times its length so that the resultant high molecular weight molecules are unidirectionally oriented. By orienting the polyvinyl alcohol matrix unidirectionally, the transition moments of the light-absorptive polyiodide become correspondingly oriented. The material thus becomes visibly dichroic.
Since the base material of "H-sheet"-type polarizers is a water-soluble high molecular weight substance, the resulting film oftentimes manifests comparatively low moisture resistance, and--in an unprotected state--tends to curl, peel, or otherwise warp when exposed to ambient atmospheric moisture. Although "H-sheet"-type polarizing film exhibits good polarizing properties (cf., a photopic dichroic ratio, R.sub.D, greater than 80), for certain applications, its moisture and heat resistance is less than desirable.
Contemporaneously with the development of the first "H-Sheet"-type polarizers, investigations of a so-called "K-Sheet" polarizer were conducted by Edwin H. Land and Howard G. Rogers at Polaroid Corporation (see U.S. Pat. Nos. 2,173,304, 2,255,940, and 2,306,108), with further development work being undertaken by F. J. Binda (see U.S. Pat. Nos. 2,445,555, 2,453,186, 2,554,850, and 2,674,159).
In contrast to "H-sheet"-type polarizers and other synthetic dichroic plane polarizers, a "K-sheet"-type polarizer derives its dichroism from the light-absorbing properties of its matrix, not from the light-absorbing properties of dye additives, stains, or suspended crystalline material. The first "K-sheet" polarizer comprised an oriented suspension of a dichroic dehydration product of polyvinyl alcohol (i.e., "polyvinylene"), in a molecularly oriented film of polyvinyl alcohol. The manufacture of such polarizer began by dehydrating sheets of polyvinyl alcohol (PVA), preferably by heating them in the presence of a very strong acid catalyst such as fuming hydrochloric acid, liberating water molecules and producing conjugated blocks of polyvinylene. Then, to effect the desired anisotropic property, the dehydrated sheet was unidirectionally stretched, aligning the conjugated polyvinylene blocks, and thus orienting the dichroic moments thereof. Due to its greater hydrophobicity (and inherent stability), early applications of such polarizers focused on optical systems where the polarizer would be subjected to high temperatures, to wide temperature fluctuations, and to extreme variations in humidity.
Although early "K-Sheet"-type polarizing film provided better heat and moisture resistance than its contemporary "H-Sheet"-type polarizing film, its optical properties were not as desirable as those of the "H-Sheet" variety. As such, early "K-Sheet" did not achieve the same degree of widespread acceptance and commercial success obtained by "H-Sheet". And accordingly, rather than improve the optical properties of early "K-sheet", subsequent efforts were more vigorously directed to improving the moisture-resistance of "H-sheet" type polarizers.
In general, two strategies were employed: one being mechanical in nature, the other chemical. With regard to the mechanical approach, water resistance in "H-sheet"-type polarizers was effected by isolating its hygroscopic material from the ambient environment in, for example, a protective envelope or laminate, see e.g., U.S. Pat. No. 4,416,946 (fluorocarbon-based moisture barrier). With regard to the chemical approach, investigations were directed to identifying and implementing hydrophobic alternatives to polyvinyl alcohol, e.g., U.S. Pat. Nos. 4,842,781, and 5,286,418; cf., U.S. Pat. No. 2,572,315, issued to J. E. Campbell on Oct. 23, 1951, and U.S. Pat. Nos. 3,621,085, 4,229,498 and 4,230,768 (dehydrohalogenation of polyvinyl chloride). With efforts weighted heavily toward the improvement of "H-sheet"-type polarizers, the investigation of "K-sheet"-type polarizers receded, and to present day, has remained virtually unpursued since the 1940s.
Currently, much commercial attention has been directed to the development and improvement of flat panel displays, and in particular, liquid crystal displays. In conjunction with the accelerated technical development of such displays, consideration is directed toward polarizers having good light-polarizing efficiency and reduced production costs. While "H-sheet"-type polarizers continue to be used in liquid crystal displays, an alternative is sought having comparable optical properties, but with moisture-resistance imparted through less costly mechanisms or approaches.